Since antibodies are protein molecules having high binding activity and binding specificity to a target molecule (antigen) and high stability in blood, applications thereof to diagnostic, preventive and therapeutic agents for various human diseases have been attempted. Although antibodies are generally produced by administering (immunizing) an antigen to a non-human animal, antibodies obtained from a non-human animal have an amino acid sequence specific to the species and side effects are caused due to that the antibodies are recognized as foreign substances in the human body. Accordingly, human chimeric antibodies or humanized antibodies have been prepared from antibodies of animals other than human (non-human animals) using gene recombination techniques.
The human chimeric antibodies and humanized antibodies have resolved problems possessed by non-human animal antibodies such as mouse antibodies, such as the high immunogenicity, low effector function and short blood half-life, and applications of monoclonal antibodies to pharmaceutical preparations were made possible by using them. In the Unites States, for example, a plurality of humanized antibodies has already been approved as an antibody for cancer treatment, and are on the market.
These human chimeric antibodies and humanized antibodies actually show effects to a certain degree at clinical level, but therapeutic antibodies having higher effects are in demand. For example, in the case of single administration of Rituxan (manufactured by IDEC/Roche/Genentech) which is a human chimeric antibody to CD20, it has been reported that its response ratio for recurrent low malignancy non-Hodgkin lymphoma patients in the phase III clinical test is no more than 48% (complete remission 6%, partial remission 42%), and its average duration of response is 12 months. In the case of single administration of Herceptin (manufactured by Genentech) which is a humanized antibody to HER2, it has been reported that its response ratio for metastatic breast cancer patients in the phase III clinical test is only 15%, and its average duration of response is 9.1 months.
The human antibody molecule is also called immunoglobulin (hereinafter referred to as Ig) and classified into subclasses of IgA1, IgA2, IgD, IgE, IgG1, IgG2, IgG3, IgG4 and IgM based on its molecular structure. The four human IgG isotypes (IgG1, IgG2, IgG3 and IgG4) are highly homologous with each other in the amino acid sequence in the H chain constant region except for the hinges showing a wide variety. However, these isotypes induce an effector activity of different strengths. In general, the ADCC activity decreases in the following order: IgG1>IgG3>IgG4=IgG2, while the CDC activity decreases in the following order: IgG3≥IgG1>>IgG2≈IgG4.
Although human IgG1 and human IgG3 are subclasses having excellent ADCC and CDC activities, it is known that human IgG3 antibody has a shorter half life in the blood than other human IgG subclasses and thus quickly disappears from the blood after the administration. It is also known that human IgG3 has no protein A-binding activity, differing from other human IgG subclasses. In producing an antibody on an industrial scale, a purification process using protein A is predominant and other processes using, for example, protein G have some problems such as a high purification cost.
Based on the above it can be said that human IgG1 antibody is the most suitable subclass as an antibody drug, since it has higher ADCC and CDC activities than other subclasses, can be purified using protein A, shows a long half life in blood and has a merit from the viewpoint of production cost. Although a human IgG1 antibody has been employed as drugs in practice as described above, the drug effects exhibited by the existing antibody drugs are still insufficient.
Thus, there has been required an antibody drug having improved effects.
Many modifications have been introduced in the amino acid sequence of the human IgG1 by swapping part of it. However, such an antibody prepared through the replacement of an amino acid sequence which is not present in the nature has a risk that it is recognized as a foreign matter in the human body and thus induces a side effect similar to the non-human animal antibody as discussed above. On the other hand, the amino acid sequence of an antibody prepared by swapping amino acid sequences between human subclasses is a combination of amino acid sequences of antibodies inherently carried by humans.